Self-calibrating MEMS bring accuracy to nanotech

This picture shows a new device called a self-calibratable MEMS. Purdue researchers have demonstrated the tiny machines, which could make possible super-accurate sensors and motors with far-reaching applications. Image: Purdue University Birck Nanotechnology Center/Jason Vaughn Clark

Researchers have demonstrated tiny machines that could make possible
super-accurate sensors and motors, with far-reaching applications from computer
storage to altimeters, detecting petroleum deposits to measuring DNA-binding
forces.

The machines are called self-calibratable micro-electromechanical systems,
or MEMS. Although MEMS are in commercial use, the new device is the first of
its kind capable of self-calibration, a step critical for applications
requiring high performance and accuracy, said Jason Vaughn Clark, an assistant
professor of electrical and computer engineering and mechanical engineering at
Purdue University.

"Self-calibration is needed because each MEMS device is slightly
different due to variations that occur in manufacturing," he said.
"Small variations in microstructure geometry, stiffness, and mass can
significantly affect performance. Because of this variability, no two MEMS
behave identically. Since conventional methods to measure MEMS properties are
usually impractical, expensive, have unknown accuracy and large uncertainty,
enabling MEMS to calibrate themselves is a game-changing innovation."

Clark previously developed the self-calibration theory. He and doctoral
student Fengyuan Li have now created the device and conducted experiments to
validate the theory. Findings are detailed in a paper to appear in the IEEE Journal of Microelectromechanical
Systems, or JMEMS.

The peer-reviewed work received a grade of A for "innovation" and
an A for "importance to the field," which testifies to the
significance of the research, Clark said.

"I think it's important to note that in 1990 MEMS pioneer Richard
Muller said research on the mechanical properties of the materials in these
devices is needed to provide the engineering base that will make it possible to
exploit fully this technology," he said. "And during a 2007 visit to
Purdue, physics Nobel laureate John Hall said that without accurate and precise
measurements, no reliable form of science or engineering is possible."

The self-calibrating technology makes it possible to accurately measure
displacement, or how far a measuring device moves, on a scale of micrometers to
less than a nanometer—a range that spans a fraction of the diameter of a human
hair to a fraction of the width of an atom.

"The difficulty in accurately measuring small displacements represents
a bottleneck in MEMS and nanotech advancements," Clark said.
"Accurate metrology is a problem that has eluded researchers since the
beginning of MEMS and nanotech in the late 1980s. Displacement is fundamental
to science and engineering. We know that quantities like velocity,
acceleration, force, stiffness, frequency, and mass can be related to
displacement. Now, using a $15 chip that can fit on your fingertip, we showed
that our technology is able to measure MEMS displacements better than a $500,000
electron microscope."

Introducing accurate measurement methods made the difference between alchemy
and chemistry about 230 years ago, and self-calibrating MEMS might bring a
similar transformation in the world if nanotechnology, he said.

"The ability to perform accurate measurements is of paramount
importance to technological advancement," he said. "In the late
1700s, Antoine Lavoisier transformed alchemy to chemistry by introducing
quantitative measurements. Today, some compare the state of micro- and
nanotechnology to alchemy, where nanotechnologists can precisely sense small
signals, but they have not had a practical way to accurately measure most
mechanical quantities. That is, no two nanotechnology labs have been able to
show that they can measure the same phenomenon and obtain the same numerical
result."

The heart of the self-calibrating MEMS are two gaps of differing size,
electrostatic sensors and tiny actuators called comb drives, so named because
they contain meshing comb-like fingers. These meshing fingers are drawn toward
each other when a voltage is applied and return to their original position when
the voltage is turned off. The comb drives measure the change in an electrical
property called capacitance while gauging the distances of the two gaps built
into the device. The fine measurements reveal the difference between the
device's designed layout and the actual dimensions.

"Once you learn the difference between layout and fabrication, you have
calibrated the device," Clark said. "Many MEMS designs with comb
drives can be easily modified to implement our technology. Our research results
suggest the days of inaccurate micro and nano-mechanical measurements are
numbered."

MEMS accelerometers and gyroscopes currently are being used commercially in
products such as the Nintendo Wii video game, iPhone, automobiles, the Segway
human transporter, and walking robots. However, those MEMS don't require
ultrahigh accuracy like those used in tactical- and navigation-grade inertial
sensors, which must undergo a complex calibration procedure in the factory. The
chips are tested using machines that translate, rotate, shock, and heat the
devices.

"The new self-calibratable MEMS could eliminate or reduce the need for
rigorous factory calibration, cutting manufacturing costs," Clark said.
"Something like 30% of manufacturing costs are related to
calibration."

The self-calibratable MEMS could lead to high-performance data storage
technologies and advanced lithography to create next generation computer circuits
and nanodevices.

Researchers will use the new self-calibration approach to improve the
accuracy of atomic force microscopes, or AFMs, which are tools essential for
nanotechnologists. Purdue operates about 30 AFMs, and Clark's research group
will use the calibrated MEMS to calibrate AFM displacement, stiffness, and
force.

The group also will use a calibrated MEMS to measure the difference in
gravity between different heights above the ground. The ability to measure
gravity with such sensitivity could be used as a new tool for detecting
underground petroleum deposits.

"Conventional gravity meters can cost over $200,000," Clark said.
"They consist of a large vacuum tube and a mirrored mass. Gravitational
acceleration is determined by measuring the drop time of the mass in free fall.
Since oil or mineral deposits have a different density than surrounding
material, the local gravity is slightly different."

The bulky and expensive gravity meters could be replaced with a small and
inexpensive MEMS chip. Another potential application is as an altimeter for
aircraft. Conventional altimeters measure height by using air pressure, which
fluctuates.

"These altimeters aren't really accurate," Clark said.
"Having a sensor that could accurately determine height would be an asset
while flying at night, through fog, or bad weather."

The self-calibratable technology also could allow MEMS to recalibrate
themselves after being exposed to harsh temperature changes or remaining
dormant for long periods.

Yet another potential application is the study of exotic phenomena such as
forces between molecules and within tiny structures on the scale of nanometers.

"A more accurate MEMS device could make it possible to measure physical
phenomena that have been beyond the resolution of conventional
technology," Clark said. "To fully understand and exploit the
attributes of the nanoscale, you really have to be able to accurately measure
subtle phenomena. Without accurate measurement tools, it becomes difficult to
discover or resolve these phenomena, to develop accurate physical models, and
to subsequently use the models to explore possibilities leading to useful
innovations."